1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, it relates to a color copying apparatus having a plurality of image bearing members and an image density control technique.
2. Related Background Art
In the past, various image forming apparatuses, and more specifically, multi-color electrophotographic copying apparatuses capable of forming a full-color image have been proposed. A typical example of such apparatuses is shown in FIG. 4 showing an elevational sectional view of a copying machine.
Within a frame C of the copying machine, four image forming portions Pa, Pb, Pc, Pd are disposed at substantially the same height, which image forming portions Pa-Pd are provided with electrophotographic photosensitive drums (image bearing members) 1a, 1b, 1c, 1d each comprising an endless electrophotographic photosensitive layer. The photosensitive drums 1a-1d which are arranged side by side are rotatably supported by the frame C of the machine in such a manner that the respective axes of the drums are parallel with each other. The drums are rotated in directions shown by the arrows by means of an appropriate drive means (not shown).
An exposure means L for illuminating color decomposed information light beams on the photosensitive drums 1a-1d is disposed above the photosensitive drums so that the light from a light source device (not shown) illuminates the photosensitive drums 1a-1d via polygon mirrors 17, thereby forming electrostatic latent images on the drums.
Further, around the photosensitive drums 1a-1d, there are disposed, in order, primary chargers 2a, 2b, 2c, 2d for uniformly charging surfaces of the photosensitive drums 1a-1d, potential sensors 22a, 22b, 22c, 22d for measuring surface potentials of the photosensitive drums 1a-1d, developing devices 3a, 3b, 3c, 3d for containing color toners (yellow toner, magenta toner, cyan toner and black toner) and adapted to develop the electrostatic latent images on the photosensitive drums 1a-1d to obtain color toner images, transfer charger devices 4a, 4b, 4c, 4d for transferring the color toner images onto a transfer sheet S (described later), cleaning devices 5a, 5b, 5c, 5d for removing the residual toners remaining on the photosensitive drums 1a-1d, and charge removing exposure devices 21a, 21b, 21c, 21d for removing charges from the photosensitive drums 1a-1d, respectively. These photosensitive drums 1a-1d are equidistantly spaced apart from each other along a feeding direction of the transfer sheet S and are designed to provide a yellow color toner image, a magenta color toner image, a cyan color toner image and a black color toner image, in that order, from an upstream side (right side in FIG. 4) of the copying machine frame C. Incidentally, the transfer charger devices 4a-4d have transfer push members 41a, 41b, 41c, 41d, respectively, which are urged against a convey belt 8 (described later).
On the other hand, below the photosensitive drums 1a-1d, there is disposed the convey belt 8 which is wound around rollers 10, 11 and is rotatingly driven by the rollers. The convey belt 8 is made from dielectric resin film such as a polyethylene terephthalate resin film sheet (PET sheet), polyvinylidene fluoride resin film sheet or a polyurethane resin film sheet, and is formed as an endless belt by connecting the respective ends of the film to each other, or as a seamless endless belt. Incidentally, in the case of a belt having a seam, a detection means may be provided for detecting the seam of the belt, so that the toner image is not transferred on the seam.
Further, a belt charge removing device 12 and a belt cleaning means 16 are arranged in a confronting relation to the convey belt 8, so that, after the any charge on the toner remaining on the convey belt 8 is removed by the belt charge removing device 12 to remove the electrostatic absorption force from the toner, the toner is removed by the belt cleaning means 16. Incidentally, the belt cleaning means 16 is constituted by a rotatable fur brush opposed to the convey belt. However, in place of the fur brush, a blade or non-woven cloth may be used, or the fur brush may be used together with such a blade or non-woven cloth. An elastic member 16a such as a brush serves to urge the belt 8 against the fur brush 16.
On the other hand, a cassette 60 containing a number of transfer sheets (cut sheets) S is mounted on the frame C of the machine at the upstream end (right side) thereof, and a pick-up roller 26 for picking up the transfer sheet S from the cassette is disposed above the cassette 60 at a downstream end thereof. A pair of register rollers 13 are disposed in the vicinity of the pick-up roller 26, which register rollers are rotatably supported by the frame C and are rotatingly driven by an appropriate drive means (not shown).
Further, a fixing device 7 is disposed within the frame C at the downstream end (left side) thereof, which fixing device has a fixing roller 71, a pressure roller 72, heat-resistive cleaning members 73, 74 for cleaning the rollers 71, 72, respectively, heaters 75, 76 for heating the rollers 71, 72, respectively, an oil applicator roller 77 for applying a mold releasing agent oil such as dimethyl silicone to the fixing roller 71, an oil reservoir 78 for supplying the mold releasing agent oil, and a fixing temperature controlling thermistor 79. The fixing device is designed to permanently fix the toner image to a transfer sheet S fed by the convey belt 8.
With the arrangement is as mentioned above, when a predetermined image forming signal is emitted, the photosensitive drums 1a-1d are rotated by the drive means (not shown), and the primary chargers 2a-2d start to uniformly charge the surfaces of the photosensitive drums 1a-1d. Further, the exposure device L exposes the photosensitive drums 1a-1d in correspondence with the respective colors to form the electrostatic latent images on the drums, and then, the developing devices 5a-5d develop the respective electrostatic latent images with the respective color toners, thereby forming the various color toner images.
On the other hand, the pick-up roller 26 is rotated in response to a predetermined signal, with the result that the transfer sheet S is fed from the cassette 60 to the interior of the frame C to be rested on the convey belt 8. Since the convey belt 8 is being rotated by the drive means (not shown), the transfer sheet S is electrostatically held on the belt and is conveyed toward the downstream side of the machine while contacting the photosensitive drums 1a-1d successively. While the transfer sheet S is in contact with the photosensitive drums 1a-1d, respectively, high voltages are applied to the transfer charger devices 6a-6d, respectively, with the result that the various color toner images on the photosensitive drums 1a-1d are transferred onto the transfer sheet S in superimposed fashion, thereby forming a full-color toner image on the transfer sheet S. Thereafter, the transfer sheet S is sent to the fixing device 7, where the full-color toner image is fixed to the transfer sheet S. Then, the transfer sheet S is discharged out of the machine. On the other hand, the toners remaining on the photosensitive drums 1a-1d are removed by the cleaning devices 7a-7d, respectively, with the result that the photosensitive drums 1a-1d can be used for the next image formation.
In a copying machine having the above-mentioned construction, image density control is effected in order to effect proper correspondence between the full-color toner image and an image on the original. Now, the image density control will be explained.
Density measuring sensors (not shown) are disposed in a confronting relation to the photosensitive drums 1a-1d, and a control means (not shown) for making the image forming condition best is incorporated within the frame C of the copying machine. In performing the density control for the toner images formed on the photosensitive drums, density measuring reference latent images are formed on the photosensitive drums 1a-1d by illuminating information light beams in response to a predetermined signal. The density measuring sensors serve to measure the density of the toner images which are obtained by visualizing the respective reference latent images with the toners, and the control means serves to control the toner density on the basis of the measured result from each density measuring sensor.
Incidentally, there are various control methods effected by the control means. Typical examples of such control methods are as follows:
(1) Method for changing a so-called "development contrast" by controlling the charging bias applied to each of the primary charges 2a-2d, an exposure light amount from the exposure device L or the developing bias applied to each of the developing devices 3a-3d;
(2) Method for altering or correcting the contents of a look-up table to ensure the density gradient in response to input video signals, in a digital copying machine; and
(3) Method for replenishing an amount of toner corresponding to the amount consumed by the developing operation to each of the developing devices 3a-3d, in a two-component developer developing system.
Among these control methods, it is known that the methods (1) and (2) can maintain the image reproductivity more stably.
By the way, in the above-mentioned conventional copying machine, even when a number of transfer sheets S are sequentially supplied to form the image on each transfer sheet, it was necessary to always maintain the density of each toner image properly by effecting a image density control method such as the above method (1), (2) or (3). Further, in the method (3) relating to the two-component developer developing system, although the toner is charged by the frictional charge between the toner and carrier particles, if a large amount of toner is replenished at a time, it takes a long time to uniformly mix the toner with the carrier particles, with the result that the frictional charge cannot be effected uniformly, and, therefore, it is feared that the image cannot be formed properly. In order to avoid this, it is necessary to effect the image density control frequently. When the image density control is effected frequently, the reference toner images must frequently be formed on the photosensitive drums 1a-1d accordingly.
On the other hand, in the digital copying machine, digital data corresponding to the density of the original image is converted in a predetermined manner, and the converted data is used as a drive signal for the exposure device L. The amount of toner consumed is calculated on the basis of the total digital data, and the image density control is effected on the basis of the calculated result. In this case, although the reference toner images are not formed frequently, the reference toner images must be formed at a predetermined interval (normally, one reference toner image per 100 copies) in order to correct the error caused by totalling the digital data.
The reference toner images so formed are contacted to the convey belt 8 as the photosensitive drums 1a-1d are rotated, and are transferred onto the convey belt 8 by the electric action of the transfer charger devices 4a-4d and the urging forces of the urging members 41a-41d. However, the more the frequency of the formation of the reference toner image is increased, the more the convey belt 8 is smudged. Although the belt cleaning means 16 is provided for removing such contamination of the belt, in some cases regarding the material (for example, polycarbonate) of the convey belt 8, the contamination of the belt cannot be fully removed by only one cleaning operation. As a result, if a transfer sheet S rests on the smudged portion of the belt, then the back surface of the transfer sheet S is smudged with the toner, which is not desirable. To the contrary, if image formation is delayed until the whole belt is completely cleaned, since the convey belt is very long, it takes a long time to complete the image forming operation, thereby reducing the image forming efficiency.